On Tue, Oct 09, 2012 at 06:55:58PM +0200, Paolo Bonzini wrote: > Il 09/10/2012 18:19, Aurelien Jarno ha scritto: > >>> > > > >> > > >> > Instead of calling the MMU helper with an additional argument (7), and > >> > then jump back (8) to the next code (4), what about pushing the address > >> > of the next code (4) on the stack and use a jmp instead of the call. In > >> > that case you don't need the extra argument to the helpers. > >> > > > Maybe it wasn't very clear. This is based on the fact that call is > > basically push %rip + jmp. Therefore we can fake the return address by > > putting the value we want, here the address of the next code. This mean > > that we don't need to pass the extra argument to the helper for the > > return address, as GET_PC() would work correctly (it basically reads the > > return address on the stack). > > > > For other architectures, it might not be a push, but rather a move to > > link register, basically put the return address where the calling > > convention asks for. > > > > OTOH I just realized it only works if the end of the slow path (moving > > the value from the return address to the correct register). It might be > > something doable. > > Branch predictors will not oldschool tricks like this one. :) >
Given it is only used in the slow path (ie the exception more than the rule), branch prediction isn't that important there. -- Aurelien Jarno GPG: 1024D/F1BCDB73 aurel...@aurel32.net http://www.aurel32.net
